The present invention relates to a stirred ball mill and a process for the fine and very fine milling of a material, namely a continuously operating stirred ball mill having a cylindrical or conical grinding chamber for receiving grinding media, a material inlet arranged at one end of the grinding chamber and opening into the interior space of the grinding chamber and a material outlet arranged at the other end of the grinding chamber and leading out of the interior space, and a stirrer which has stirring members, is coaxial with the chamber axis and is intended for moving the grinding media, the stirring members being paddlewheel-like or propeller-like and having a plurality of transport members.
Stirred ball mills are used, for example, for the comminution and dispersing of solids in a liquid phase or for the digestion of microorganisms.
Stirred ball mills which are known on the market and intended for continuous operation have a cylindrical or conical grinding chamber which possesses a horizontal or vertical axis and whose housing is formed from a longitudinal wall and two end walls arranged at the two ends of the longitudinal wall. One end wall has a material inlet which serves for feeding in the mill base and the other end wall has a material outlet which serves for removing the material.
Furthermore, a stirrer which is rotatable about the chamber axis and serves for transporting the grinding media present in the grinding chamber radially relative to the stirrer shaft and thus to comminute, by means of impact and shear forces, the material passed continuously through the grinding chamber, is arranged in the grinding chamber. For this purpose, the stirrers installed in the known stirred ball mills have a plurality of stirring members capable of rotating coaxially with the chamber axis. Said stirring members are, for example, in the shape of a disk having an essentially circular circumference and are divided into tapering prongs extending in the tangential direction by slots extending to the circumference.
During the continuous operation of known stirred ball mills having horizontal and vertical axes, the grinding media are entrained in the direction of the material outlet by the stream of the material to be milled, with the result that the grinding media accumulate in the region of the material outlet and lose their milling and dispersing effect because they are no longer moved by the stirring members of the stirrer. The grinding media accumulating in the region of the material outlet during the milling and dispersing process hinder the continuous discharge of the product stream, resulting in an increase in the internal pressure of the chamber. This increase in the internal pressure of the chamber finally results in the grinding media and also the stirring members being subjected to relatively severe mechanical wear during the milling and/or dispersing process and in the necessity of increasing the stirring power required to move the stirring members in order to maintain a product stream flowing uniformly out of the grinding chamber. The accumulation of grinding media in the region of the material outlet furthermore results in an increase in the grinding temperature, which may also have an adverse effect on the quality of the product to be produced.